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Setup and Configure an OpenVPN Server on CentOS 6

July 14th, 2014 — whplus

This article will guide you through the setup and configuration of OpenVPN server on your CentOS 6 cloud server. We also cover how to configure openvpn client to connect to your newly installed OpenVPN server.

Before we begin, you’ll need to have the Extra Packages for Enterprise Linux (EPEL) Repository enabled on your cloud server. This is a third party repository offered by the Fedora Project which will provide the OpenVPN package.

Now that we have the file in the proper location, open it for editing:

nano -w /etc/openvpn/server.conf

Our first change will be to uncomment the “push” parameter which causes traffic on our client systems to be routed through OpenVPN.

push "redirect-gateway def1 bypass-dhcp"

We’ll also want to change the section that immediately follows route DNS queries to Google’s Public DNS servers.

push "dhcp-option DNS 8.8.8.8"
push "dhcp-option DNS 8.8.4.4"

In addition, to enhance security, make sure OpenVPN drops privileges after startup. Uncomment the relevant “user” and “group” lines.

user nobody
group nobody

Generating Keys and Certificates Using easy-rsa

Now that we’ve finished modifying the configuration file, we’ll generate the required keys and certificates. As with the configuration file, OpenVPN places the required scripts in the documentation folder by default. Create the required folder and copy the files over.

In order to allow clients to authenticate, we’ll need to create client certificates. You can repeat this as necessary to generate a unique certificate and key for each client or device. If you plan to have more than a couple certificate pairs be sure to use descriptive filenames.

Do not forget to replace 123.123.123.123 with your server IP.
Now save that iptables rules:service iptables save

Then, enable IP Forwarding in sysctl:

nano -w /etc/sysctl.conf

# Controls IP packet forwarding
net.ipv4.ip_forward = 1

Finally, apply our new sysctl settings. Start the server and assure that it starts automatically on boot:

sysctl -p
service openvpn start
chkconfig openvpn on

You now have a working OpenVPN server. In the following steps, we’ll discuss how to properly configure your client.

Configuring OpenVPN Client

Now that your OpenVPN server is online, lets configure your client to connect. The steps are largely the same regardless of what operating system you have.

In order to proceed, we will need to retrieve the ca.crt, client.crt and client.key files from the remote server. Simply use your favorite SFTP/SCP (Secure File Transfer Protocol/Secure Copy) client and move them to a local directory. You can alternatively open the files in nano and copy the contents to local files manually. Be aware that the client.crt and client.key files will are automatically named based on the parameters used with “./build-key” earlier. All of the necessary files are located in /etc/openvpn/easy-rsa/keys

With our certificates now on our client system, we’ll create another new file called client.ovpn, where “client” should match the name of the client being deployed (from build-key), the contents should be as follows, substituting “x.x.x.x” with your cloud servers IP address, and with the appropriate files pasted into the designated areas. Include only the contents starting from the “BEGIN” header line, to the “END” line, as demonstrated below. Be sure to keep these files as confidential as you would any authentication token.

As all of the required information to establish a connection is now centralized in the .ovpn file, we can now deploy it on our client system.

On Windows, regardless of edition, you will need the official OpenVPN Community Edition binaries which come prepackaged with a GUI. The only step required post-installation is to place your .ovpn configuration file into the proper directory (C:\Program Files\OpenVPN\config) and click connect in the GUI. OpenVPN GUI on Windows must be executed with administrative privileges.